The use of mechanical actuation systems on aircraft is becoming more wide spread as the advantages of synchronized multiple actuators become better known. To date however, the use of mechanical actuation systems appears to be limited to non-critical flight surfaces. The aircraftindustries' hesitance to use such mechanical actuation systems appears to be rooted in the basic fear that the mechanical actuator can jam and lock up a surface. In the prior art, if this fear is realized and the actuator jams, the entire system will lock up since the actuators are mechanically connected together. On the one hand we see a benefit in the actuators being mechanically connected together in that it allows the desirable end of distributing the load equally into all of the actuators while on the other hand a single jammed actuator locks up the system into an inoperative condition.
In the prior art the two most commonly employed mechanical actuators for use in flight control surface movement are the rotary gear box and the linear ball screw. Of these two types of actuators the rotary gear box seems to be the type that creates the most concern in respect of the matter of jamming.
If a flight control surface or panel as it may be termed, such as a rudder panel, were to jam in a hard-over condition, the effect on the aircraft's manuverability would be profound. While it is possible to fly an aircraft with a non-operating rudder panel or for that matter any other non-operating flight control surface, in many cases it is not possible to operate an aircraft when one of the flight control surfaces are in the hard-over condition.
In the prior art there are a number of patents that are directly concerned with the detection of the failure of a shaft in an actuation system. One of these patents is that of Comollo, U.S. Pat. No. 3,935,754 which detects and indicates the failure of the primary drive train of an aircraft flap actuation system that includes primary and secondary drive trains driven by a common power supply. When the primary drive train fails, a free motion zone formed between the common power supply and the secondary drive train is crossed. Crossing of the free motion zone causes the position of an electro-mechanical sensor(s) to change. The change in position of the electro-mechanical sensor(s) creates or changes the value of an electical signal or signals to provide an indication of the failure of the primary drive train.
The patent to Comollo does not provide, as does the invention to be described hereinafter, an arrangement that will allow for continued operation after a jam type failure as distinguished from a shaft failure.
Another patent thought to be of interest is that of Maltby, U.S. Pat. No. 3,986,689 which patent in addition to providing a shaft failure detection circuit arrangement 73, 74, 77 in FIG. 3, also teaches a flap actuation system where one or more ball screw mechanisms are employed to move the flaps simultaneously on the wings of an aircraft.
In Maltby torque to actuate the ball screw mechanisms is normally transmitted through a primary load path connecting a power drive unit and the ball screw mechanism. In the event that there is a failure in the primary load path, a back-up load path connected between outer ends of the primary path is available to deliver power to the ball screw mechanism so that symetrical positioning of the control surfaces of the aircraft may still be achieved. Connection to the opposite ends of the primary load path, enables torque to be transmitteed through the back-up shaft from either end. Accordingly, should there be a failure, for example, in a middle segment of the primary power path on one side of the aircraft, torque will be transmitted through the shaft of the back-up load path from the opposite side of the aircraft to the point of failure thereby providing power to all of the remaining ball screw mechanisms on the one side of the aircraft from the opposite direction.
The Maltby patent suffers from the same deficiencies as does the Comollo patent in that Maltby does not contemplate, as does the invention to be described hereinafter, the simultaneous operation of three or more flaps or surfaces where one of the surfaces becomes jammed and the remaining surfaces can then be operated to provide some form of control of the aircraft.
A final patent of interest is that of Embree, U.S. Pat. No. 4,256,277 which shares an assignee in common with the inventor of this application. Embree is directed to an actuation system for an aircraft control surface and particularly to an actuator system designed to prevent a symmetric deployment of the control surfaces in the event of a failure of a primary power transmitting component of the system.
In FIG. 1 of Embree it will be observed that to assure symmetric deployment of flap panels 11 on opposite sides of the aircraft in the event of failure in one of the shaft segments 24, a back-up load path is defined by a shaft 55 extending between opposite outer ends 54 of the two outer primary load path segments 24. Both ends of the shaft 55 are driven in the same direction and at the same speed during normal operation of the system so that the shaft is unloaded, except for frictional losses, thereby giving it an essentially infinite load life in comparison to the components of the primary load path. The back-up shaft 55 is spaced separately from the longitudinal axes of the primary load path. Whenever there is a failure in the primary load path 22, torsional deflection of the shaft is sensed and a signal produced which indicates a failure.
The invention of this specification is not primarily concerned with shaft failure, but as noted herein before, the invention permits continued control of the aircraft by utilizing the remaining flight control surfaces when one of the surfaces has become jammed.